1. Field of the Invention
The present invention relates to an LED lamp including an LED chip, which is covered with a resin portion containing a phosphor, and also relates to a method for fabricating such an LED lamp.
2. Description of the Related Art
White LED lamps are recently under vigorous research and development as potential replacements for white incandescent lamps. In some of those white LED lamps, the package of a blue LED chip, made of gallium nitride (GaN), is coated with a phosphor such as YAG. In such an LED lamp, the blue LED chip produces an emission with a wavelength of about 450 nm, and the phosphor produces yellow fluorescence with a peak wavelength of about 550 nm on receiving that emission. Eventually, the emission and fluorescence mix with each other, thereby providing white light.
In another type of white LED lamp currently under development, an LED chip that emits an ultraviolet ray is combined with a phosphor that produces red (R), green (G) and blue (B) light rays. In such an LED lamp, the ultraviolet ray that has been radiated from the LED chip excites the phosphor, thereby emitting the red, green and blue light rays. Consequently, white light can also be obtained as a mixture of these light rays.
A bullet-shaped package is extensively used in conventional LED lamps. Hereinafter, such an LED lamp with a bullet-shaped appearance will be described with reference to FIG. 1.
FIG. 1 illustrates a cross-sectional structure for a conventional LED lamp 20 as disclosed in Japanese Patent No. 2998696, for example. As shown in FIG. 1, the LED lamp 20 includes an LED chip 21, a bullet-shaped transparent housing to cover the LED chip 21, and leads 22a and 22b to supply current to the LED chip 21. A cup reflector 23 for reflecting the emission of the LED chip 21 in the direction indicated by the arrow D is provided for the mount portion of the lead 22b. The inner walls (i.e., reflective surfaces) of the cup reflector 23 surround the side surfaces of the LED chip 21 so as to define a predetermined tilt angle with respect to the bottom of the cup reflector 23. The LED chip 21 on the mount portion is encapsulated with a first resin portion 24, which is further encapsulated with a second resin portion 25.
The first resin portion 24 is obtained by filling the cup reflector 23 with a resin material and curing it after the LED chip 21 has been mounted onto the bottom of the cup reflector 23 and then has had its cathode and anode electrodes electrically connected to the leads 22a and 22b by way of wires. A phosphor 26 is dispersed in the first resin portion 24 so as to be excited with the light A that has been emitted from the LED chip 21. The excited phosphor 26 produces fluorescence (which will be referred to herein as “light B”) that has a longer wavelength than the light A. This LED lamp 20 is designed such that if the light A radiated from the LED chip 21 is red, then the light B emitted from the phosphor 26 is yellow. A portion of the light A is transmitted through the first resin portion 24 including the phosphor 26. As a result, light C as a mixture of the light A and light B is used as illumination.
The conventional LED lamp shown in FIG. 1, however, has a color unevenness problem.
In this LED lamp, the light C, obtained by mixing the light A and light B together, is used as illumination as described above. Accordingly, color unevenness is easily created in the light C depending on the shape of the first resin portion 24 including the phosphor 26.
In the conventional LED lamp 20, the first resin portion 24 is obtained by filling the cup reflector 23 with a resin material and curing it such that the LED chip 21 is encapsulated with the resin material. Thus, the shape of the first resin portion 24 is defined by that of the internal recess of the cup reflector 23. In the LED lamp 20 shown in FIG. 1, the reflective surfaces of the cup reflector 23 are tilted so as to define a downwardly tapered cross section. Accordingly, the upper surface of the resultant first resin portion 24 is broader than the lower surface thereof, and the side surfaces thereof make tight contact with the reflective surfaces of the cup reflector 23. That is to say, the cup reflector 23 is closely filled with the first resin portion 24 so as to create no gaps between the first resin portion 24 and the cup reflector 23.
Specifically, the first resin portion 24 is obtained by pouring a resin liquid into the cup and curing it. For that reason, the upper surface of the first resin portion 24 often becomes uneven as shown in FIG. 1. In addition, since the resin portion 24 has a downwardly tapered cross section, the upper surface of the resin portion 24 has a relatively broad area, thus creating significant effects. That is to say, such unevenness on the upper surface of the first resin portion 24 makes the thickness of the resin layer including the phosphor uneven. In that case, the amount of the phosphor included in one part of the resin portion 24 will be significantly different from that of the phosphor included in another part of the resin portion 24. In other words, the amount of the phosphor included changes according to the optical path of the light A being transmitted through the resin portion 24. As a result, quite noticeable color unevenness is created in the light C.
Furthermore, since the first resin portion 24 makes close contact with the reflective surfaces of the cup reflector 23, the part of the first resin portion 24 surrounding the side surfaces of the LED chip 21 has non-uniform, variable thicknesses. In that case, the light that has gone out of the LED chip 21 through a side surface thereof is absorbed into the phosphor in the first resin portion 24 in variable amounts while being transmitted through the first resin portion 24 and before reflected from the reflective surfaces. The amount of the light absorbed into the phosphor also changes with the optical path thereof because the thicknesses of that part of the resin portion 24 are non-uniform. FIG. 2 schematically shows the optical paths E and F of the light that has been radiated through a side surface of the LED chip 21. As can be seen from FIG. 2, when taking the optical path E, the light A needs to go a relatively short distance through the first resin portion 24. On the other hand, when taking the optical path F, the light A needs to go a relatively long distance through the first resin portion 24. The light A radiated from the LED chip 21 is absorbed into the phosphor while exciting the phosphor and making the phosphor radiate the light B. Accordingly, if the light A radiated from the LED chip 21 should go different distances through the first resin portion 24, then the mixture ratio of the light A and light B is changeable with the optical path. As a result, significant color unevenness is created in the light C for use as illumination. The optical path length is often variable if the side surfaces of the first resin portion 24 are tapered to reflect the internal shape of the cup reflector 23 as shown in FIG. 2.